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Antarctica
Antarctica is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctica region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km² (5.4 million sq mi), it is the fifth-largest continent in area after Asia, Africa, North America, and South America. About 98% of Antarctica is covered by ice, which averages at least 1.6 kilometers (1.0 mi) in thickness. Geology Geological history and paleontology More than 170 million years ago, Antarctica was part of the supercontinent Gondwana. Over time, Gondwana gradually broke apart and Antarctica as we know it today was formed around 25 million years ago. Paleozoic era (540–250 Ma) During the Cambrian periodic stage, Gondwana had a mild climate. West Antarctica was partially in the Northern Hemisphere, and during this period large amounts of sandstones, limestones and shales were deposited. East Antarctica was at the equator, where sea floor invertebrates and trilobites flourished in the tropical seas. By the start of the Devonian period (416 Ma), Gondwana was in more southern latitudes and the climate was cooler, though fossils of land plants are known from this time. Sand and silts were laid down in what is now the Ellsworth, Horlick and Pensacola Mountains. Glaciation began at the end of the Devonian period (360 Ma), as Gondwana became centered around the South Pole and the climate cooled, though flora remained. During the Permian period, the plant life became dominated by fern-like plants such as Glossopteris, which grew in swamps. Over time these swamps became deposits of coal in the Transantarctic Mountains. Towards the end of the Permian period, continued warming led to a dry, hot climate over much of Gondwana. Mesozoic era (250–65 Ma) As a result of continued warming, the polar ice caps melted and much of Gondwana became a desert. In East Antarctica, the seed fern became established, and large amounts of sandstone and shale were laid down at this time. The Antarctic Peninsula began to form during the Jurassic period (206–146 Ma), and islands gradually rose out of the ocean. Ginkgo trees and cycads were plentiful during this period, as were reptiles such as Lystrosaurus. In West Antarctica, coniferous forests dominated through the entire Cretaceous period (146–65 Ma), though Southern beech began to take over at the end of this period. Ammonites were common in the seas around Antarctica, and dinosaurs were also present, though only two Antarctic dinosaur genera (Cryolophosaurus, from the Hanson Formation, and Antarctopelta) have been described to date. It was during this period that Gondwana began to break up. Gondwanaland breakup (160–23 Ma) The cooling of Antarctica occurred stepwise by the continental spread changing the oceanic currents from longitudinal equator-to-pole temperature-equalizing currents to latitudinal currents that preserved and accentuated latitude temperature differences. Africa separated from Antarctica around 160 Ma, followed by the Indian subcontinent, in the early Cretaceous (about 125 Ma). About 65 Ma, Antarctica (then connected to Australia) still had a tropical to subtropical climate, complete with a marsupial fauna. About 40 Ma Australia-New Guinea separated from Antarctica, so that latitudinal current could isolate Antarctica from Australia, and so the first ice began to appear. Around 23 Ma, the Drake Passage opened between Antarctica and South America, which resulted in the Antarctic Circumpolar Current. The ice spread, replacing the forests that then covered the continent. Since about 15 Ma, the continent has been mostly covered with ice, with the Antarctic ice cap reaching its present extension around 6 Ma. References Category:Antarctica Category:Continents